This invention relates generally to automatic transmissions mounted on motor vehicles and, more particularly, to an automatic transmission capable of being suitably mounted on a front engine front drive type small vehicle which requires a reduction in the size of the transmission. More specifically, the present invention relates to a hydraulic controller for changing over the speed of an automatic transmission with a band brake.
Conventionally, in ordinary automatic transmissions, a controller operates a one-way clutch at the time of shifting to a predetermined transmission speed to prevent occurrence of a shift shock caused by clutching changeover of two frictional engagement elements (e.g., a clutch and a brake).
A type of controller which operates no one-way clutch at the time of shifting to a predetermined transmission speed, e.g., a controller which releases a band brake while engaging a clutch is known. In this controller, the timing of supplying a clutch engaging pressure or a band brake releasing pressure produced from the clutch engaging pressure is controlled by means of an orifice control valve.
Another type of controller is known which includes a release valve for simultaneously operating two frictional engagement elements. The release valve is controlled by a solenoid valve so that when one of two frictional engagement elements is released, the other is engaged simultaneously (refer to Japanese Laid-Open Patent No. 62-31741).
However, these types of hydraulic controllers for automatic transmissions entail drawbacks described below. The hydraulic controller using a one-way clutch can prevent occurrence of a shift shock caused by clutching changeover of the frictional engagement elements but requires a multiplicity of one-way clutches for enabling smooth shifting in addition to a multiplicity of clutches and brakes for ordinary use. It therefore makes it difficult to reduce the overall size of the automatic transmission particularly in the axial direction. In some case, the automatic transmission cannot be mounted on a small motor vehicle in which the automatic transmission mount space is restricted.
In the case of the controller which controls the timing of supplying clutch engaging pressure or band brake releasing pressure by using an orifice control valve, the automatic transmission size can be reduced. However, the range of changes in the throttle opening or vehicle speed in which the supply timing can be changed over is limited, and it is difficult to change over the supply timing with respect to the whole range of changes; there is a possibility of occurrence of a large shift shock. The orifice control valve necessitates a complicated control process which increases the cost of the automatic transmission. This controller is also unsatisfactory in terms of reliability because there is a possibility of the supply timing being shifted by hydraulic pressure dispersion or changes in frictional characteristics and viscosity with respect to time.
The controller using a solenoid valve and a release control valve for timing control necessitates a complicated and expensive electrical control system. It is therefore difficult to apply this type of controller to automatic transmissions for small vehicles manufactured under various restrictions.